The present invention relates to a method for storage and recovery of thermal energy or, in particular, to a novel method for storage and recovery of thermal energy utilizing a medium substance for heat storage and transfer, which is not used hitherto in such a purpose, in which thermal energy can be stored as latent heat in the form of chemical energy and, when releasing of the heat is desired, the latent heat is readily and conveniently converted to sensible heat.
The invention also relates to a heat pump in which transfer of heat is effected by the above mentioned medium substance.
In recent years, there is a world-wide trend or demand for effective utilization of various thermal energies such as waste heat and solar energy as a heat source, which have been considered to be of low value owing to their diffused availability. In order to utilize these thermal energies with efficiency and convenience, it is necessary to develop a method that the thermal energy obtained from the heat source is concentrated and stored in a medium which is transported to the place where the thermal energy is utilized and the thermal energy is released as sensible heat when required.
There have been proposed various kinds of substances suitable for use as a heat-storage medium, among which one of the most promising classes utilizes the reversible exothermic and endothermic reactions in the combination reaction of reactants or decomposition reaction of a chemical compound where the thermal energy is converted into chemical energy and temporarily stored in the medium substance.
In particular, most of the prior art heat-storage method utilizes a solid compound as the medium which is decomposed endothermically with evolution of a gaseous product leaving a solid product which can be regenerated into the initial form with recovery of the thermal energy as released sensible heat.
These solid heat-storage media are defective since they are disintegrated into powder by the repeated cycles of evolution of a gas and absorption of the same gas so that the effective thermal conductivity rapidly decreases to an extent that the absorption and releasing of the thermal energy are extremely suppressed in the practical use of the medium substance. This problem is more serious when large scale use of the medium is intended. What is worse, a solid medium is inconveniently transported, for example, by use of a pump in comparison with liquid heat-storage medium substances and, when a combustible or explosive gaseous product such as hydrogen is involved in the reaction, there can be a danger of fire or explosion.
Accordingly, there has been eagerly desired to develop a heat-storage medium with high efficiency, in which large capacity for heat storage, good thermal conductivity and easiness in transportation as well as safety in handling are the key factors.